NGC 3603-A1 is a massive, double-eclipsing binary star system located in NGC 3603, about 20,000 light years from Earth. Its two component stars circle each other every 3.77 days. The mass of NGC 3603-A1a is 116 ± 31 solar masses and 89 ± 16 solar masses for NGC 3603-A1b.[2] This makes them the two most massive stars directly measured so far, i.e. their masses have been determined (using Keplerian orbits), and not estimated. Both show an emission-line spectrum (spectral type WN6h). A separate study resolved the pair and individually modelled their masses at 120 and 92 M ☉.